You can also enter “villa delenia” into google maps, and find it – use “Nea Styra” as your start off point, not Athens airport, or it will send you the long way, via the Chalkida Bridge, rather than the ferry.
Once you turn right off Marathon Avenue, do not go into the village of Marathon, but stay on the same road for about 6 km until you reach the village of Kato Souli - you will actually go into this village. At the village crossroads, you will see a sign saying Ferry Boat, so turn left at that point. After about 3 kms, you will see another sign, which says Ferry Boat, indicating a right turn - this is the only slightly tricky bit of the journey, as it is easy to drive past that sign - just look out for a taverna (traditional greek restaurant) on the left hand side of the road, and a traditional white village house on a corner, opposite the taverna and on the right hand side of the road - turn right on that corner, and then you are just a few minutes away from the Aegean sea and the port of Agia Marina.
When you arrived at Nea Styra: If you enter "villa delenia" into google maps, it takes you there BUT there is one bit to avoid, towards the end of your journey. When you get to a white cabin in the village of Elaiochori it will tell you to turn left down a very steep and narrow road - IGNORE THAT! Just keep on the 'main road' (which is also quite narrow) and drive around the church/cemetery, with it on your left, NOT on your right!